This invention relates to a method and system for increasing the quantity of dissolved gas in a liquid and maintaining a substantial amount of the increased quantity of dissolved gas in solution until the gas/liquid solution is delivered to a point of use. More particularly, the invention relates to a method and system of increasing the quantity of dissolved gas in a liquid by using pressurized mixing and delivery of the gas and the liquid. Furthermore, by subjecting the gas/liquid solution to controlled dispensing at a point of use, a substantial quantity of the increased quantity of gas is maintained in solution.
Ozone has long been recognized as a useful chemical commodity valued particularly for its outstanding oxidative activity. In fact, ozone is the fourth strongest oxidizing chemical known, having an oxidation potential of 2.07 volts. Because of this property, ozone and/or fluid mixtures including ozone are capable of removing a wide variety of contaminants, such as cyanides, phenols, iron, manganese, and detergents, from surfaces. Also, ozone and/or fluid mixtures including ozone are capable of oxidizing surfaces. In particular, ozonated water is used to xe2x80x9ccleanxe2x80x9d, i.e., oxidize, the surface of silicon wafers in-process in the semiconductor industry. Additionally, ozone is also useful for inhibiting, reducing and/or eliminating the accumulation of biomass, mold, mildew, algae, fungi, bacterial growth and scale deposits in various aqueous solution systems. When used in this manner, ozonation provides the advantage of producing a lesser quantity of potentially harmful residues than, e.g., chlorination, which leaves undesirable chlorinated residues in aqueous systems.
Because of this wide range of activity, ozone finds application in many diverse processes. Ozone, for example, has been used as a biocide for the treatment of drinking water. Additionally, ozone is used for sterilization in the brewing industry, and for odor control purposes in the sewage treatment industry. Finally, ozonated water finds wide utility in the semiconductor industry, where for example, ozone is used to clean and surface condition in-process silicon wafers. Additionally, as is described in U.S. Pat. No. 5,378,317, ozonated water is used to remove organic materials, such as photoresist, from the surface of silicon wafers. Moreover, ozonated water is used in the semiconductor industry to form a thin, passivating oxide layer on the surface of silicon wafers.
The use of ozonated water provides several advantages in these applications. First of all, because ozonated water is generated at the point of use, it is free of contaminants, i.e., particles and metals, that are typically present in chemicals that-are stored in barrels or drums. Ozonated water is also less expensive than other oxidizing chemicals and furthermore, since ozonated water naturally decomposes, the use of ozonated water presents no disposal issues. However, the effectiveness of ozone in each of these applications is adversely affected by its low solubility and short-half life (approximately 10 minutes) in aqueous solutions. That is, not only is it difficult to dissolve ozone in an aqueous solution, but also, once dissolved, it is difficult to maintain the ozone in solution.
Although several methods of increasing the quantity of dissolved ozone in aqueous solutions are known, each of these prior art methods has limitations that render them inadequate for certain applications. For example, bubbling ozone directly into water at ambient pressure has been used as a method to dissolve ozone in aqueous solutions. Such a technique, however, does not optimize the quantity of ozone dissolved, since the ozone bubbles effervesce before a substantial amount of ozone can be dissolved into solution and/or before the ozonated water can be applied to the surface to be treated.
Additionally, published European patent application No. EP 0 430 904 A1 discloses a process for producing ozonated water comprising the step of contacting, within a vessel of defined volume, an ozone-containing gas with fine droplets of water. However, this process is less than optimal since it provides limited contact between the ozone-containing gas and water. That is, as the vessel fills with water, the time of contact between the ozone containing gas and the fine water droplets is shortened, resulting in a lesser quantity of ozone being dissolved into solution. Additionally, this application does not teach a method of keeping the ozone in solution until it is delivered to a point of use. Thus, it is possible that, upon delivery, a large quantity of the ozone dissolved in solution will effervesce, and the benefits of the mixing process will be lost.
Finally, several methods utilizing cooling to increase the quantity of dissolved ozone in aqueous solutions have also been proposed. For example, U.S. Pat. No. 5,186,841 discloses a method of ozonating water comprising injecting ozone through an aqueous stream across a pressure drop of at least 35 psi. The ozonated stream is then combined with a second stream that is preferably a portion of an aqueous solution which is recirculating in a cooling water system. The resultant stream is forced to flow at a velocity of 7 feet per second for a distance sufficient to allow 70% of the ozone to be absorbed. Additionally, U.S. Pat. No. 4,172,786 discloses a process for increasing the quantity of dissolved ozone in an aqueous solution by injecting an ozone containing gas into a side stream conduit which circulates a portion of cooling water. The ozone-injected water is then mixed with the cooling water in a tower basin, thereby ozonating the water. Finally, U.S. Pat. No. 5,464,480 discloses a process for removing organic materials from semiconductor wafers using ozonated water. Specifically, this patent teaches that high ozone concentration water, suitable for use in the disclosed process may be obtained by mixing ozone and water at a temperature of from about 1xc2x0 C. to 15xc2x0 C.
Although the systems disclosed in U.S. Pat. Nos. 5,186,841, 4,172,786 and 5,464,480 claim to increase the quantity of dissolved ozone in water, it is more likely that much of the ozone effervesces to the atmosphere and/or is converted to oxygen rather than being dissolved in the water. Thus, these systems would require the use of a large amount of ozone, which would, in turn, render them costly. Additionally, these patents do not disclose methods for optimizing the ozone concentration at the point of use, and as a result, it is possible that the increased ozone, if any, that is dissolved as a result of cooling the solution, will effervesce out of solution at the point of use.
Thus, there is a need for an efficient method of increasing the quantity of ozone that may be dissolved and maintained in aqueous solution to a point of use, not only to minimize the amount of ozone used, but also to provide sufficiently ozonated aqueous solutions for given applications.
According to the present invention, the above objectives and other objectives apparent to those skilled in the art upon reading this disclosure are attained by the present invention which is drawn to a method and system for increasing the quantity of dissolved gas in a liquid and for optimizing the amount of dissolved gas that remains in solution to a point of use. More specifically, it is an object of the present invention to provide a method and system for increasing the quantity of dissolved ozone in an aqueous solution, and furthermore, for maintaining the dissolved ozone in solution when delivered to a point of use. In this manner, the present invention provides an exceptionally efficient method and system for producing and using high concentration ozonated water.
Generally, the method involves introducing a stream of a gas to be dissolved into a pressurized vessel wherein the gas is contacted with, and dissolves in, an amount of liquid. Mixing the gas to be dissolved with the liquid under pressure results in an increased amount of gas being dissolved, relative to the amount of dissolution that occurs at atmospheric pressure. The resulting admixture comprising the liquid and the dissolved gas is then delivered to a point of use through a pressurized conduit, which maintains the increased amount of dissolved gas in solution.
At the point of use, the admixture is subjected to controlled dispensing. That is, the admixture is dispensed under sufficiently gentle conditions such that the dispensed admixture comprises a supersaturated quantity of dissolved gas at the time the admixture contacts the substrate. Preferably, the admixture is dispensed under conditions such that the resulting delivered volume of admixture has a relatively small surface area/volume ratio. By virtue of the small surface area/volume ratio, the amount of diffusion of the dissolved gas out of the liquid is limited, thus maintaining an increased concentration of dissolved gas in the liquid to a point of use. In contrast, when the surface area/volume ratio is relatively large, the dissolved gas diffuses out of solution more quickly. Thus, dispense methods which result in delivered volumes of admixture with a large surface area/volume ratio also result in delivered volumes with a decreased concentration of dissolved gas at the point of use.
Several methods of controlled dispensing are suitable for use in the present invention. For example, at the point of use, the admixture may be subjected to controlled atomization. Specifically, the admixture may be atomized under conditions such that the average size of the resulting droplets is large (i.e., the surface area/volume ratio is small) relative to the size of droplets created by conventional atomization. That is, conventional atomization typically produces a fine mist, i.e., small droplets with a relatively large surface area/volume ratio. Thus, the dissolved gas will diffuse out of solution more quickly resulting in a decreased concentration of dissolved gas at the point of use. In contrast, the controlled atomization utilized in the method and system of the present invention results in large droplets with a smaller surface area/volume ratio, thus limiting the amount of diffusion that takes place and maintaining an increased concentration of dissolved gas in solution to a point of use. Controlled atomization may be effected by a number of mechanisms. For example, in a preferred embodiment, the admixture may be xe2x80x9cgentlyxe2x80x9d impinged with either a second stream of admixture or a stream of inert gas, e.g., nitrogen, in a manner that results in the desired droplet size.
The controlled dispensing may also be effected by delivering the admixture through a fan structure at the point of use. When controllably dispensed in this manner, the fan nozzle breaks the stream of admixture into smaller sheets or large droplets of admixture, thus resulting in delivered volumes of admixture with the desired small surface area/volume ratio.
Yet another example of a controlled dispensing method suitable for use in the present invention includes gently dispensing the admixture as a steady stream. In this embodiment of the invention, the admixture may be dispensed directly to the point of use, e.g., the surface of a silicon wafer, or alternatively, the admixture may be gently dispensed into a suitably sized vessel, i.e., a vessel with a small open surface area, but yet a large volume. That is, the desired small surface area/volume ratio may be achieved simply by gently dispensing the admixture into a vessel with suitable dimensions so as to result in the desired surface area/volume ratio.
In a preferred embodiment, the liquid utilized in the method and system of the present invention is a fluorinated liquid, sulfuric acid, hydrochloric acid, hydrofluoric acid, water, ultrapure deionized water or combinations thereof. More preferably, the liquid utilized is water or ultrapure deionized water. Additionally, the method and system of the present invention are applicable to a variety of cleaning gases, including, but not limited to, hydrogen chloride, nitrogen, carbon dioxide, oxygen, hydrogen fluoride, ammonium hydroxide, ozone or combinations thereof. In a particularly preferred embodiment, the method and system are used to increase the quantity of dissolved ozone gas in ultrapure deionized water.
As a result of the ability of the method and system of the present invention to increase and maintain the quantity of dissolved cleaning gas in a liquid, the resulting admixtures are expected to be particularly useful in the treatment of various surfaces. In particular, ozonated water prepared with the method and/or system of the present invention is effective to clean, i.e., oxidize and/or remove organic contaminants and/or photoresist materials, from surfaces such as in-process silicon wafers. In this regard, the present invention also provides a method for treating surfaces with a cleaning gas. Specifically, the method comprises the steps of preparing an admixture comprising a cleaning gas dissolved in a liquid within a pressurized vessel and transferring the admixture to an outlet through a pressurized conduit. The admixture is then dispensed through the outlet under sufficiently gentle conditions such that the dispensed admixture comprises a supersaturated quantity of dissolved gas at the time the admixture contacts the substrate.
The system provided by the present invention generally comprises a pressurized vessel and an outlet coupled to the pressurized vessel adapted to dispense a stream of the admixture comprising the liquid and the dissolved gas under sufficiently gentle conditions such that the dispensed admixture comprises an increased quantity of dissolved gas relative to admixture produced and dispensed by conventional methods. In one preferred embodiment, the outlet comprises a spray post comprising at least one fixed orifice located at a suitable point in a treatment vessel (e.g., a wet bench) such that admixture may be gently dispensed thereinto. Preferably, the treatment vessel is of dimensions that result in a small surface area/volume ratio of the dispensed admixture, so that diffusion of the gas out of the liquid is minimized. In a second preferred embodiment, the outlet comprises a spray post comprising a single fixed orifice located a suitable distance from a point of use such that a steady stream of admixture may be gently dispensed thereonto. In a third embodiment, the spray post may comprise a plurality of one or more sets of fixed orifices distributed along at least one surface of the spray post at suitable intervals to effect the atomization of a stream of the admixture by impingement with at least a second fluid stream. The second fluid stream preferably may either be a gas stream or a second stream of the admixture. Finally, in a fourth preferred embodiment, the outlet may comprise a spray post comprising a plurality of fan structures distributed along at least one surface, the fan structures being effective to break up a stream of the admixture into sheets and/or large droplets with the desired small surface area/volume ratio.
Preferably, the system further comprises a liquid sensing device operatively coupled to the pressurized vessel, a liquid source responsive to the liquid sensing device, a gas source capable of delivering a generally continuous supply of gas to the pressurized vessel, a pressurized liquid outlet conduit fluidly coupled to the pressurized vessel and a pressurized gas outlet conduit through which undissolved gas can exit the pressurized vessel. Additionally, in a preferred embodiment, the pressurized gas outlet conduit can be used to restrict the flow of undissolved gas out of the pressurized vessel, thus maintaining pressure in the pressurized vessel and aiding in the motivation of admixture from the pressurized vessel through the pressurized liquid outlet conduit. In a preferred embodiment, the pressurized vessel also comprises an amount of a flow impediment effective to increase the residence time of the gas in the pressurized vessel. For example, the pressurized vessel may comprise packing material, such as a fluorinated polymer, quartz, sapphire or combinations thereof, or baffles.
As used herein, the term xe2x80x9caqueousxe2x80x9d means any fluid admixture that contains water as a solvent, including impure water. The term xe2x80x9csupersaturatedxe2x80x9d is meant to indicate that a liquid contains a greater amount of a dissolved constituent than is present in a saturated solution of the same components at the same temperature and pressure. As used herein, the term xe2x80x9cozonatedxe2x80x9d means that ozone is dissolved in a given liquid. The phrase xe2x80x9cultrapure deionized waterxe2x80x9d, as used herein, is meant to indicate water that has been treated by filtering, reverse osmosis, and UV sterilization so as to remove particles, metals and organics, respectively. The phrase xe2x80x9ccontrolled dispensingxe2x80x9d or xe2x80x9ccontrollably dispensedxe2x80x9d is meant to indicate a method of dispensing admixture under sufficiently gentle conditions such that the dispensed admixture comprises a supersaturated quantity of dissolved gas at the time the admixture contacts a substrate. Preferably, the phrase xe2x80x9ccontrolled dispensingxe2x80x9d or xe2x80x9ccontrollably dispensedxe2x80x9d is meant to indicate a method of dispensing that results in a delivered volume of admixture of a sufficiently small surface area/volume ratio so that the increased quantity of dissolved gas is maintained in solution until delivery to a point of use. The phrase xe2x80x9cgenerally continuous supplyxe2x80x9d as applied to a gas source is meant to indicate a gas source capable of generating a gas from suitable precursors or a gas source such as tanks, cylinders, and the like, for use in a steady state process as opposed to a batchwise process. Finally, the phrase a xe2x80x9ccontinuous processxe2x80x9d refers to a process that can be operated by supplying input materials and withdrawing output materials under substantially steady state conditions after start-up and prior to shutdown.